Not only do a third of North Americans battle hay fever – with runny noses, sinus and eye symptoms – but for a significant proportion of allergy sufferers, the spring bloom is just the start of their allergy woes. Dr. Antony Ham Pong, a Canadian allergist and researcher, estimates that up to 10 per cent of the general population has a condition called oral allergy syndrome, or OAS.

It’s a less severe form of food allergy, directly related to pollen reactions, that’s known to set off tingling and unpleasant itching in the mouth, throat and lips. Reactions are caused by a variety of fruits, vegetables, nuts, seeds, and even spices; all of which share allergenic proteins with specific hay fever-causing trees and plants.

“It’s not considered a life-threatening allergy,” says Ham Pong, the author of several articles educating patients and doctors about oral allergy syndrome. “But it’s actually more common than peanut, milk, egg, and fish allergy.”

Two of the biggest cross-reaction offenders are birch and alder trees. Depending on where you live, anywhere from 20 to 70 per cent of people who are allergic to birch and alder pollens will also have oral allergy syndrome.

Ham Pong estimates about one-third of birch-allergic North Americans are affected, but the incidence of oral allergy syndrome is even higher in some European countries. Although OAS is relatively common, he doesn’t think it is increasing, at least not in North America. Rather, doctors have become better at spotting this condition.

But how can a tall, skinny tree that gives you the sniffles cause an itchy mouth if you chew on a celery stick or bite into an apple?

How the immune system gets confused

Unfortunately for those with tree-pollen allergies, the pollen proteins that trigger hay fever symptoms are also found in a several foods.

The components of those proteins are well conserved among different plants, which means their three-dimensional structures are so similar that some of our immune systems can’t tell the difference between, say, birch pollen and apple peel.

In birch, the most common allergy troublemaker is a protein called Bet v 1, and alder has a similar protein called Aln g 1. Their purpose is to help protect the fruit or vegetable in times of stress or from infection.

Ham Pong explains that Bet v 1 is called a “pan-allergen” because very similar proteins are found in a variety of unrelated plants, including apples, plums, kiwis, carrots, celery, potatoes, hazelnuts, almonds, sunflower seeds, and even spices such as oregano, basil and dill.

People with birch-related oral allergy syndrome already produce antibodies to the pan-allergens from their prior exposure to birch pollen, so those antibodies will try to protect against similar proteins when they enter the mouth.

Pan-allergen relationships don’t stop with birch trees. The allergenic components of grass pollen are also shared with melons, oranges, kiwi, tomatoes and peanuts, among others.

For those reactive to ragweed, serious cross-reactions are also possible from consuming chamomile, honey and echinacea. This is not oral allergy syndrome, however. In this case, it’s because they belong to the same botanical family.

Apples and Peaches and Oral Allergy

The better news for spring is that although trees such as maple, oak and poplar produce allergenic pollens, they don’t share reaction-causing proteins with foods. Nor does having allergies to birch, alder, grass or ragweed mean you’ll necessarily cross-react with the implicated foods.

And, if you do react, you may not be allergic to every food on the list.

Some fruits and vegetables, however, are more troublesome. Apples, for instance, more often trigger oral allergy syndrome than pears. According to Japanese research, in the plum family, cherries and peaches are more likely to irritate than apricots and plums.

It may even depend on which variety of the fruit you eat. European studies, for example, have found that gala and golden delicious apples have higher levels of allergenic proteins than Braeburn or Santana apples, and are more likely to cause reactions.